Effective therapies for acute leukemias have been developed incrementally, advanced by the development of agents with different mechanisms of action. Sapacitabine, the orally bioavailable form of CNDAC (2'-Ccyano- 2'-deoxy-1-/3-D-ara[unreadable]>/no-pentofuranosyl- cytosine), is a potent cytosine nucleoside analogue with a novel mechanism of action. The design of CNDAC was based on the concept that once its triphosphate is incorporated into DMA, the addition of a subsequent deoxynucleotide would initiate 3-elimination, resulting in cleavage of the 3'-5'phosphodiester linkage and conversion of incorporated analogue to a chain-terminating nucleotide, CNddC. Clinically active in phase I studies, the mechanisms of action of sapacitabine are unique among therapeutic agents, as it causes a single strand nick in DMA that is terminated by a nucleotide that cannot be extended and is resistant to repair. Our hypothesis is that sapacitabine will elicit novel pharmacodynamic responses for detecting DMA damage, for repair of the lesions, and for activation of cell cycle checkpoints that will serve as biomarkers to guide clinical development of sapacitabine alone and in combination with targeted inhibitors of these pathways. We will conduct translational studies in primary leukemia cells in vitro and during therapy that will complement and extend ongoing laboratory investigations. The specific aims for testing this hypothesis are: 1. Evaluate pharmacokinetics and pharmacodynamics of sapacitabine during phase l/ll clinical trials in patients with relapsed/refractory leukemias, 2. Characterize biomarkers for cellular responses to sapacitabine, 3. Formulate laboratory rationales for clinical translation of mechanism-based combinations of sapacitabine with small molecule inhibitors. The overall goal of our proposal is aimed at developing a thorough understanding of cellular responses to sapacitabine that will identify biomarkers that have prognostic value to optimize patient selection and schedules of administration in the clinic, and to provide rationales for combinations with agents targeted at inhibiting DMA damage sensors, DNA repair mechanisms, and dysregulating checkpoint controls. Lay abstract: We have identified a drug candidate that kills cancer cells in ways that are different from all other drugs. We have shown that it active treatment in humans with leukemia who have not responded to standard therapies, and propose laboratory and clinical studies that may increase its effectiveness at treating these and other cancers.